1. Field of the Invention
The present invention relates generally to polyethylene production and, more specifically, to polyethylene particle size in the operation of a polyethylene polymerization reactor system having two or more polymerization reactors.
2. Description of the Related Art
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyethylene polymer and its copolymers are used for piping, retail and pharmaceutical packaging, food and beverage packaging, plastic bags, household items, various industrial products, and so forth.
Polyethylene may be produced from the monomer ethylene. If the sole monomer ethylene is used for polymerization, the polyethylene polymer is referred to as a homopolymer, while incorporation of different monomers in addition to ethylene creates a polyethylene copolymer or terpolymer, and so on. In polyethylene production, the comonomer 1-hexene is commonly used in addition to ethylene to control density of the polyethylene. The monomers (ethylene, 1-hexene, etc.) may be added to a polymerization reactor, such as a liquid-phase reactor or a gas-phase reactor, where they are converted to polymers. In the liquid-phase reactor, an inert hydrocarbon, such as isobutane, propane, n-pentane, i-pentane, neopentane, and/or n-hexane, may be utilized as a diluent to carry the contents of the reactor. A catalyst (e.g., Ziegler-Natta, metallocene, chromium-based, etc.) may also be added to the reactor to facilitate the polymerization reaction. Unlike the monomers, catalysts are generally not consumed in the polymerization reaction.
As polymer chains develop during polymerization, solid particles known as “fluff” or “flake” or “powder” are produced. The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), comonomer content, molecular weight, and so on. Different properties for the fluff may be desirable depending on the application to which the polyethylene fluff or subsequently pelletized polyethylene fluff is to be applied. Control of the reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, catalyst type, and so forth, may affect the fluff properties.
In some circumstances, to increase capacity of a polymerization line or to achieve certain desired polymer characteristics, the polymerization conditions may benefit from employing more than one polyethylene polymerization reactor, with each reactor having its own set of conditions. The reactor conditions, including the polymerization recipe, can be set and maintained such that polyethylene polymer product is monomodal, bimodal, or multimodal. In the case of bimodal or multimodal polymers, at least two polyethylene polymers, each having a different molecular weight fraction, for instance, may be combined into one polymer product. In a general sense, a polyethylene produced in each reactor will be suspended in a diluent to form a slurry. The reactors may be connected in series, such that the slurry from one reactor may be transferred to a subsequent reactor, and so forth, until a polyethylene polymer is produced discharging from the final reactor with the desired set of characteristics. For example, a bimodal polymer may be produced by two reactors in series, a trimodal polymer may need three, and so on.
The competitive business of polyethylene production drives manufacturers in the continuous improvement of their processes in order to lower production costs, improve product quality, and address environmental concerns, and so on. In an industry where billions of pounds of polyethylene product are produced per year, small incremental improvements, such as in reactor stability and operability, monomer and diluent recovery, and the like, can result in significant economic benefit and environmental progress, and so forth.